Ultrasonographic identification of the cricothyroid membrane: best evidence, techniques, and clinical impact  M.S. Kristensen, W.H. Teoh, S.S. Rudolph  British Journal of Anaesthesia  Volume 117, Pages i39-i48 (September 2016) DOI: 10.1093/bja/aew176 Copyright © 2016 The Author(s) Terms and Conditions

Fig 1 The transverse (‘Thyroid cartilage–Airline–Cricoid cartilage–Airline=TACA’) method for identification of the cricothyroid membrane. First row: the transducer is placed transversely on the neck where the thyroid cartilage is thought to be until the triangular shape of the thyroid cartilage (=the ‘T’) is identified. Second row: the transducer is moved caudally until the ‘Airline’ (=the ‘A’) is seen; this is the hyperechoic (white) line from the tissue–air border on the luminal side of the cricothyroid membrane. The white line has similar echo lines deep to it; those are reverberation artefacts. Third row: the transducer is moved further caudally until the cricoid cartilage (=the ‘C’) is seen as a black lying ‘C’ with a posterior white lining. The white lining represents the tissue–air border on the luminal side of the anterior part of the cricoid cartilage. Forth row: subsequently, the transducer is moved a few millimetres back in the cephalad direction, and the approximate centre of the ‘Airline’ (=the ‘A’=the cricothyroid membrane) is thus identified and can be marked with a pen. British Journal of Anaesthesia 2016 117, i39-i48DOI: (10.1093/bja/aew176) Copyright © 2016 The Author(s) Terms and Conditions

Fig 2 Upper row: step (i) The patient is lying supine, and the operator stands on the patient's right side facing the patient. The sternal bone and the suprasternal notch are palpated, which can be done in even the morbidly obese patient. The linear high-frequency transducer is placed transversely over the neck just cranial to the suprasternal notch, and the trachea is seen in the midline. The middle and the right photographs show the ultrasound image, and the relevant structures are highlighted on the photograph to the right. Blue is the anterior part of a tracheal ring. Yellow indicates the tissue–air boundary inside the trachea. Everything below the tissue–air boundary is artefact. Second row: step (ii) The transducer is slid laterally towards the patient's right side, until the midline of the trachea is at the right border of the transducer, and the corresponding ultrasound image of the trachea (in the right photograph) is truncated into half. Blue is the anterior part of a tracheal ring. Yellows indicate the tissue–air boundary inside the trachea. Third row: step (iii) Staying midline with the right edge of the transducer, the left edge of the transducer is rotated into the sagittal plane to obtain a longitudinal image of the trachea. The anterior parts of the tracheal rings appear as black hypoechoic round structures (like pearls) lying on a strong hyperechoic white line, which is the tissue–air boundary (that looks like a string). Hence, the image is akin to a ‘string of pearls’. The blue markings represent the anterior parts of the tracheal rings. Yellow indicates the tissue–air boundary inside the trachea. Fourth row: step (iv) The transducer is slid cephalad, and the anterior part of the cricoid cartilage (turquoise) is seen as a slightly elongated structure that is significantly larger and more anterior than the tracheal rings (blue). Yellow indicates the tissue–air boundary inside the trachea. Immediately cephalad to the cricoid cartilage is the cricothyroid membrane. The distal part of the thyroid cartilage (purple) is seen. Step (v) The cricothyroid membrane can be pointed out by sliding a needle (used only as a marker) underneath the ultrasonography transducer from the cranial end until it casts a shadow (red line) between the cricoid cartilage (turquoise) and the thyroid cartilage. The green spot represents the reflection from the needle. Care is taken not to touch the patient with the sharp tip of the needle. Step (vi) After this, the transducer is removed, and the needle indicates the mid-level of the cricothyroid membrane. This spot can be marked with a pen so that it remains easily identified should it be needed for subsequent difficult airway management. (The figure in the last row is from Kristensen and colleagues).11 British Journal of Anaesthesia 2016 117, i39-i48DOI: (10.1093/bja/aew176) Copyright © 2016 The Author(s) Terms and Conditions